A detector of charged particles, such as a photomultiplier (PM) or a secondary electron multiplier (SEM) is typically used in instruments such as a mass spectrometer. These detectors offer ground-referenced electrical pulse signals (which are termed “events”), which may float on a high voltage reference (as high as 2 kV) and are normally of negative polarity due to the charge of electrons. In a mass spectrometer, such detection signals can then be used to create a mass spectrum (indicating the relative abundance of ions of different masses), for example by counting the number of events received within a discrete period of time. Improving the detection accuracy and discrimination of events therefore provides a higher quality mass spectrum.
The detected pulses can have durations of several nanoseconds. Where the duration of a pulse is in the range of 5 ns to 20 ns, these coincide with noise and interference from bursts on the mains power supply, radio and digital bus signals in the frequency domain. To reduce this interference problem, the output of the detection electrode can be ground-isolated, for example by use of a signal transformer. This mitigates the problem of DC and low frequency AC interference on the ground loop and can offer a significant improvement in Signal-to-Noise Ratio (SNR) of the measured signal.
The use of a transformer can have further advantages, in that polarities can be changed and impedances can be matched. Moreover, a transformer can act to filter out noise and interference that lie in the stop-band of the device. This may enhance signal quality.
However, such arrangements can also have difficulties. The pulse signals originating from the detection electrode are short transients. This means that the isolated signal, especially when generated using a transformer, can comprise additional artefacts. In particular, the transient part of the pulse can comprise an additional over-shoot or under-shoot, not present in the original signal. As a flux-coupled transformer can only pass AC signals, this kind of distortion may be created in the isolated signal.
Such distortion means that the pulse duration is made less distinct. Not only does this reduce the precision of any mass spectrum generated using this signal, but this distortion can also mask an immediately adjacent pulse and therefore make it difficult to distinguish pulses. Improving the quality of the detector output signal is therefore a challenge.